Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3409
Ibn Abbas (Allah be pleased with them) reported: It was proposed that he (the Holy Prophet) be married to the daughter of Hamza, whereupon he said: She is not lawful for me for she is the daughter of my foster-brother, and that is unlawful by reason of fosterage what is unlawful by reason of genealogy.